1. Field of the Invention
The present invention relates to a filter circuit suitable for a band-pass filter arranged on a rear stage of a power amplifier in a transmitter of a radio communication apparatus.
2. Description of the Related Art
In a transmitter of a radio communication apparatus, a band-pass filter is arranged on a rear stage of a power amplifier which amplifies a high frequency signal to supply a transmission power to a radio antenna. Such a filter is realized generally by connecting a plurality of resonators in cascade. In this case, if input and output coupling coefficients of a resonator and a value of an external Q are properly determined, it is possible to determine a passage frequency range of the filter and a blocking domain attenuation quantity thereof.
The power of a signal input to the filter passes all resonators connected in cascade with approximately the same electric energy. Energy (power) stored in each resonator depends upon input and output coupling coefficients of the resonator. The input coupling coefficient is a coupling coefficient between the input terminal of the resonator and an input circuit, and the output coefficient is a coupling coefficient between the output terminal of the resonator and an output circuit. Generally, the power to be stored in the resonator concentrates on the resonator that the input and output coupling coefficients are small. A problem for power concentration is that concentration of electric fields on a metallic edge heats the metal due to the resistance thereof, resulting in burning dielectrics used for decreasing the size of the filter. Because it is difficult in design to change the resonator according to electric field concentration, and to exchange a resonator, a filter is made using a resonator of high allowable over-power.
Consequently, for the purpose of realizing a desired filter property by distributing a signal power passing the filter, a method of fabricating a filter by connecting a plurality of resonators in parallel is provided by Japanese Patent Laid-Open No. 2001-345601. This filter divides the power to plural resonators. Also, this filter increases a group delay on both ends of a required bandwidth similarly to a cascade-connected resonator type of filter. The parallel connection of the plural resonators divides an input signal power into the resonators, resulting in improving power-resistance property of the filter. In this case, the resonators have different resonance frequencies, and are arranged so that signals passing the resonators having adjacent resonance frequencies are in opposite phase to each other. As a result, a desired filter property can be realized.
The filter fabricated by connecting a plurality of resonators in parallel is designed so as to be equivalent to a filter fabricated by a plurality of resonators connected in cascade as described in Kato, Yamanaka, Ma, and Kobayashi, “Study of an equivalent circuit of double mode rectangular waveguide filter using HFSS and MDS” Singaku Jihou, MW 98-85, pp. 73-80, September 1998. This filter realizes a desired filter property by changing input and an output coupling coefficients between resonators. For this reason, the power distribution cannot uniformly done.
A modulation system used for radio communication in recent years is an angle modulation system such as QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation), and signal components are included in phase information. Therefore, a band-pass filter provided in a transmitter can uniformly divide a power to resonators to flat a group delay characteristic caused by phase distortion.
It is an object of the present invention to provide a filter circuit capable of performing uniformly power dispersion to a plurality of resonators connected in parallel and realizing flat group delay characteristic, and a radio communication apparatus using the same.